The subject of the present invention is a device for controlling the field current of DC motors which are operated beyond the normal rotational speed in the higher speed range attainable by weakening of the field, there being provided for the apportioning of the field current a phase angle control device which is operated in dependence on a control variable which is obtained from the measured speed by means of a function-forming device as a function (precontrol characteristic) of the rotational speed.
The typical DC motor consists in principle of two parts, a field magnet with a field winding which, when traversed by a field current, produces a stationary magnetic field, and a rotating armature. The armature has a plurality of windings which are connected to the terminals of the motor via a commutator and via brushes. Cooperating with the magnetic field produced by the current-carrying field winding, the current-carrying armature generates the forces that bring about the rotation of the armature. The commutator serves for successive current supply to the individual windings of the armature during rotation of the armature, and this in such a way that the direction of current flow in the respective windings of the armature remains in the same position relative to the poles of the field magnet circuit. By the rotation of the armature windings in the magnetic field there is produced in the armature, according to the law of induction, a countervoltage (EMF) which is opposed to the applied armature voltage.
In the stationary case of motor operation, exactly that rotational speed will occur at which the EMF induced in the armature is equal to the armature voltage reduced by the ohmic voltage drop caused by the current flow in the armature circuit. The relationship between the rotational speed n, the induced EMF, and the machine flux MF caused by the field current is represented by the well known basic formula of the DC machine resulting from the law of induction: EQU EMF =k MF n,
where k is a machine constant.
In principle, two operating ranges may be distinguished in the control of DC machines, namely a range in which the machine field is kept at the maximum value, the "nominal field", and the speed increase is achieved by increasing the armature voltage, and a second range, the so-called "field weakening range", in which to obtain higher speeds the machine field is weakened starting from the nominal field. It has been found to be desirable to weaken the exciting field in such a way that the EMF induced in the armature of the DC machine remains constant in the latter range, namely at the desired value ES.
As the armature voltage must remain limited to the rated armature voltage of the motor, when operating in the first range a limit speed is reached which is referred to as "base speed". The base speed corresponds to the speed at which, for maximum instantaneous load of the motor corresponding to the rated armature current and for rated armature voltage, the rated power is obtained; the then occurring EMF equals the desired value ES. The desired value ES thus results from the rated armature voltage minus the ohmic armature voltage drop at the rated armature current.
The power delivered at the shaft of the DC motor is (expressed in mechanical quantities) equal to the product of rotational speed and torque or respectively (expressed in electrical quantities) equal to the product of EMF and armature current.
If operation of the motor above the base speed is desired, it becomes necessary to weaken the field, as a further increase of the rated armature voltage after reaching the other maximum rated values of the motor is not permitted. Now if it is to be possible to deliver the rated power also in the field weakening range, it follows that the EMF of the machine is constant and that the magnetic flux must be reduced in inverse proportion to the rotational speed. Reduction of the magnetic flux as a result of a reduction of the field current is connected with a proportional reduction of the available maximum torque. There are many practical cases for DC machines which manage with a reduced torque, but which require higher rotational speeds than the base speed.
Modern controls for DC motors, therefore, make use of the counter-EMF as a reference quantity for obtaining a ratio between magnetic flux and rotational speed. Being that the magnetic flux and the rotational speed are to give a constant product, a reciprocal relationship results between these two quantities. For the magnetic flux control in the field weakening range, therefore, there results a hyperbolic relationship between magnetic flux and rotational speed, such that with increasing speed the magnetic flux decreases in inverse proportion to the speed.
A control device of this kind is described in U.S. Pat. No. 4,549,122, where this hyperbolic characteristic curve is illustrated in FIG. 7, and which is realized there by means of an analog computation circuit which sets a desired value of the field current inversely proportional to the speed. However, because of the non-linearity of the magnetization characteristic, which may be individually different for each motor, a linear relationship does not exist in reality between the field current and the magnitude of the magnetic flux. This is set forth in column 8 of the U.S. patent, in lines 59 to 68. In FIG. 7, the (idealized) characteristic curve based on a linear relationship between field current and magnetic flux is represented by a bold solid curve. The characteristic adapted to actual requirements, on the other hand, is shown as a dashed curve. The adapted characteristic is obtained by adjusting of resistors (56 and 57 in FIG. 4), which is expressed there by the word "program" (in line 65). This, however, does not involve true programming, but only a shift of the characteristic. Hence only characteristics can be obtained in this way which belong to a given single-parameter family of curves. More specific characteristics, which require several variable parameters for their representation, cannot be obtained by means of the circuit described in U.S. Pat. No. 4,549,122, or only approximately so.